1. Field of the Invention
This invention relates to a vertical axis wind turbine, and particularly to an arrangement and installation method of blades that can effectively improve the efficiency of a vertical axis wind turbine.
2. Description of the Related Art
To make better use of wind energy, wind power generation devices have been developed. These can be divided according to the different position of the rotary axis of the wind turbine into horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs).
VATW rotors can be also divided into two types: lift-type rotors and drag-type rotors. Characteristics of lift-type rotors and drag-type rotors are described, e.g., in Wind energy and its utilization, Beijing: China Energy Press, 1984. Although both lift-type and drag-type rotors are vertical axis rotors, the principles behind their working are completely different.
In lift-type rotors, when wind blows over the surface of a blade, wind speeds at the blade's outer and inner surfaces are different due to different profile of the surfaces. The difference in wind speeds creates pressure difference between the surfaces and hence a lift force. Because the blades are installed in such a way that the angle of attack varies from blade to blade, a driving moment is created around rotor's centre of gyration, driving the rotor to rotate. However, when the wind rotor rotates, a specific resistance is produced by the blades due to their wing shape. The higher the rotating speed, the bigger the resistance, until a balance is achieved between the lift force and resistance. Therefore, the efficiency of the lift-type wind rotor depends on its design parameters, including the wing shape and the angle of installation of the blades.
Drag-type rotors differ from lift-type rotors in that the shape of the blades is different and in that in drag-type rotors the drag coefficients are different on both sides of the blades. The side with a bigger drag coefficient obtains a bigger wind pressure. Because drag-type rotors use the component of wind's force that is vertical to wing surface, “positive work” is done in rotor's right half circumference, while “negative work” is done in the rotor's left half circumference. Because the wind speeds at left and right sides are the same and the difference only lies in the drag coefficients on both sides of the blade, the work done in the left and the right half circumferences is a function of the blade's drag coefficient multiplied by the cube of the blade's relative wind speed.
If the wind speed is denoted by V and the linear speed of the rotor rotation is denoted by u, then because the wind “drives” blade to move in the right half circumference, the blade's relative linear speed is V−u, and because the blade moves upwind in the left half circumference, the blade's relative linear speed is V+u. Therefore, when the wind blows from the left, the left gate flap has the biggest moment and the moments at other positions are smaller. Once the blades are selected, the drag coefficients on both sides of blades are fixed. Therefore, the difference between work done in the rotor's left and right half circumferences decreases with the increase of rotor's rotational speed, i.e., the efficiency of the drag-type rotor decreases with the increase of the rotor's rotational speed. This is opposite to the efficiency profile of lift-type rotors.
The wind rotor of lift-type vertical axis wind turbine adopts wing-shaped straight blades. The axis line of the blade is parallel to that of the center of gyration. Normally, the blade is fixed on a blade support rotating around the center of gyration. When a strong enough wind blows over straight blades that are wing-shaped and form a specific angle with the tangent of rotating axis, lift force and resistance are produced. When the lift force is higher than the resistance, a moment of rotation around vertical axis is produced on these blades. However, the size and direction of the lift force and resistance changes continuously due to the constant changes in the rotating angle of blades with respect to the wind direction during rotation. That is to say that the size and direction of a moment produced at different positions of blades constantly changes. At some positions, a positive moment is produced, at other positions, a negative moment is produced. Thus, the wind energy utilization ratio of vertical axis wind turbine is decreased.
In aerodynamics, the rotating angle between chord line of the blade and the tangent of a position in the circumference is called the blade rotating angle α. The azimuth angle of a wing-shaped blade at any relative position in the circumference is defined as β. The connecting line between blade's front and rear edges is called the chord line L, and the radius of the wind rotor's rotating around vertical axis is called rotation the radius of the wind rotor R. The ratio of the rotation radius of the wind rotor to the chord line of the blade is the radius/chord ratio. The radius/chord ratio is a very important parameter in the installation of the blades and a wind rotor of vertical axis wind turbine.
In existing designs of lift-type vertical axis wind turbines, there has been no emphasis on optimization of the radius/chord ratio. Conventionally, the size of the blade was set to be very narrow, i.e., the radius/chord ratio is set to be very large. This resulted in a starting difficulty but allowed the wind rotor to attain a high rotating speed. That is to say that the torque produced by the wind rotor was very small. Alternatively, the radius/chord ratio was set to be very small, which allowed for an easy rotor starting but resulted in a large resistance produced on the blade during the rotation of the wind rotor at a very high wind speeds; thus, the wind rotor was unable to reach higher rotating speeds.
The power of the rotor equals torque multiplied by the angular speed. Therefore, the conventional designs could neither make the motor obtain a large enough torque nor rotate at a sufficiently high speed. This affects the wind energy utilization ratio of vertical axis wind turbine and lowers the commercial value of conventional vertical axis wind turbines.
Besides, in conventional designs, blades are usually directly connected to the support arm by welding or by bolts and nuts. But, in an actual wind field, the force impact on the wind rotor consisting of blades is very complicated and in particular, a very strong centrifugal force is produced during high-speed rotation. Thus, the safety of blades connected to the support arm using the above methods cannot be ensured and blade fracture or detachment from the support arm may occur. In addition, existing vertical axes are all coaxially connected with the motor. But the wind rotor encounters a very large force impact in an actual wind field, which inevitably produces a horizontal vibration on the vertical axis, and thus adversely effects the operation of the rotor.